Fiber optic technology has now been used in telecommunications for several decades. In high-bandwidth applications, optical fiber has virtually replaced old-fashioned copper wire. The reasons for this transition are many. Optical fiber's advantages over copper wire include wide bandwidth, low signal attenuation, light weight, immunity to electromagnetic interference, absence of electrical sparking, virtual elimination of crosstalk, physical flexibility, small size, and low cost.
In optical networks, an electrical signal to be transmitted modulates light, typically of the infrared variety with a wavelength in the 1.3 or 1.5 μm bands. The modulated light is transmitted over an optical fiber, received, and demodulated to recover a copy of the original electrical signal.
A significant number of fiber-optic networks is intended for the commercial transport of data. As such, the network operator seeks wide bandwidth capability at low cost, with high reliability, fidelity, efficiency, and security. The transmission bandwidth of a single fiber is quite high. Cost-effective systems that transport data at 10 Gbits/s (OC-192) and 40 Gbits/s (OC-768) on a single wavelength channel are presently available, and the transmission rate capability will likely continue to grow. But the total throughput theoretically realizable from a fiber is much higher, several tens of Terahertz. Dense wavelength-division multiplexing (DWDM) systems have evolved to use more of this available spectrum. In such systems, many different wavelength channels are used concurrently to transmit data-carrying signals over the same fiber. Systems using up to 160 different wavelengths that carry as much as 10 Gbit/s each and provide a total capacity of 1.6 Tbit/s in a single fiber have been developed. For fiber links requiring less capacity, a less dense architecture, called coarse wavelength-division multiplexing (CWDM), with fewer channels at more widely spaced wavelengths, may be used in similar fashion. DWDM and CWDM will be collectively referred to as wavelength-division multiplexing, or WDM.
In WDM optical networks, it is necessary to multiplex (combine) multiple independent wavelengths when transmitting, and demultiplex (separate) those wavelengths back into individual channels when receiving. Multiplexing and demultiplexing functions are performed by optical filters, typically thin-film interference filters, waveguide interferometers such as arrayed-waveguide gratings (AWG's), or fiber Bragg gratings. The nodes of an optical network typically provide both types of functions, extracting some or all of the individual wavelength channels on one or more inbound fibers, and recombining some of those wavelengths with new wavelengths carrying data from local sources into one or more outbound fibers. Optical filters that perform these and other functions are distributed abundantly throughout a WDM optical network.
Significant costs are associated with converting data from optical to electrical representation, or from electrical to optical representation. Therefore, it is usually preferred that data that is not bound to or from the local traffic of a node be passed through the node transparently, that is with no conversion or non-linear processing. Because of the transparency, a given optical signal may pass through several optical filters on its route through the network. To avoid excessive degradation of signal fidelity, the precision and stability of the optical filter bandshapes and phase responses need to be more stringently controlled in a distributed network than for direct connections.
AWG's are fabricated from integrated planar waveguides and scale elegantly to higher channel counts. Although they are the most recently developed of the commonly-applied filter architectures, AWG's are beginning to dominate applications where eight or more optical channels are being multiplexed or demultiplexed, because of their high performance and cost-effectiveness. FIG. 1 illustrates a typical arrayed waveguide grating 10 with an input waveguide 12, one or more output waveguides 14, a phased array of waveguides 16 arranged side-by-side, and a pair of focusing slab regions 18 and 19. The focusing slab region 18 couples light from the input waveguide 12 into the near end of the waveguides of the phased array 16, while the focusing slab region 19 couples the light emerging at the far end of the waveguides of the phased array 16 into the output waveguides 14. The waveguides of the phased array 16 have different lengths, with a constant optical path-length difference ΔLop between neighboring waveguides.
When monochromatic light illuminates the input waveguide 12, the slab 18 spreads the energy of the light into the near ends of the waveguides 16a-16n. Because of the differing optical path-lengths of the waveguides 16a-16n, the light emerging at the far ends of the waveguides 16a-16n has different phases, depending on the particular waveguide traveled by the light. After the emerging light is combined by the focusing slab region 19, the different phases interfere constructively at some point, depending on the wavelength of the light. If the point is coincident with one of the output waveguides 14, the light is coupled into this optical waveguide. Changing the wavelength of the monochromatic light changes the point of constructive interference, moving it from one output waveguide (e.g., 14a) to another (e.g., 14b).
As is known from linear system theory, a non-monochromatic WDM light signal having a spectral distribution of
      ∑    i    ⁢            c      i        *          λ      i      can be treated simply as a collection of individual monochromatic signals ci*λi. Therefore, if the input waveguide 12 is illuminated with this WDM signal, each of its constituent wavelength components λi will focus at a different point and will be coupled into a different output waveguide 14. The constituent wavelength channels λi of the WDM signal are thus physically separated—i.e., demultiplexed—into different output waveguides 14. Through reciprocity principle applicable to electromagnetic fields, if a wavelength λj incident on the input waveguide 12 is channeled into an output waveguide 14j, then the same wavelength channel λj incident on the output waveguide 14j will be channeled—i.e., multiplexed—into the input waveguide 12. The arrayed waveguide grating 10 can thus be used as both a multiplexer and a demultiplexer, depending on which end is selected as the input.
Planar waveguide technology used for fabricating AWG's is an optical integration technology. As optical network nodes become more and more complex, the AWG becomes just one of multiple optical circuit elements integrated onto a chip. Examples of devices having higher scales of integration include multiplexer/demultiplexer matched pairs, add-drop multiplexers, programmable wavelength blockers, and wavelength-selective routing switches. There are of course other integrated optical circuits that may include one or more arrayed waveguide gratings.
One way of more stringently controlling the precision and stability of the optical filter bandshapes and phase responses of AWG's is to control the center wavelength of the AWG. The center wavelength λc of an AWG depends on the phase shifts through the phased waveguides of the AWG. (“Center wavelength” means the wavelength that is optimally or near optimally channeled into a particular output waveguide.) The phase shifts depend on the refractive index of the waveguides, which in turn depends on the temperature of the waveguides. For silica-based waveguides, the magnitude of the temperature coefficient of the refractive index
  (            ⅆ      n              ⅆ      T        )is of the order of 10−5/(° C.), or 10 parts per million per degree Celsius, which translates into a similar magnitude of the wavelength temperature coefficient
      (                  ⅆ        n                    ⅆ        T              )    ,and into a temperature frequency coefficient
  (            ⅆ      f              ⅆ      T        )of about 2 GHz/(° C.) at 1.5 μm wavelength. For channel spacing of about 100 GHz and less, operation of the arrayed waveguide grating over the commercial temperature range of even 60° C. is problematic. Moreover, without expensive trimming it is difficult to manufacture arrayed waveguide gratings having sufficient center wavelength accuracy for the relatively narrow channel spacing. For this reason, arrayed waveguide gratings and similar optical components are center wavelength-stabilized.
Most commonly this stabilization is accomplished by actively regulating the temperature of the optical chip. According to this method, a heating element uniformly elevates the temperature of the component to a set-point temperature. Varying the selection of the set-point temperature allows tuning of the grating to a chosen center wavelength, eliminating or decreasing the need for trimming and increasing yields.
In one variation on the basic method of thermally tuning an arrayed waveguide grating, an integrated heater is patterned to cover longer lengths of some waveguides of the phased array than of other waveguides of the array. If the shape of the heater is properly chosen, localized heating generated by the operation of the patterned heater causes an increase in the effective optical path-length differences between neighboring waveguides. In other words, it causes tilting of the waveform to increase (or decrease) by a higher factor than would have resulted from a heater uniformly distributed over the array's waveguides. Because the patterned heater affects the phased waveguides unevenly, the sensitivity of the center wavelength to temperature of the heater increases, and the tuning range of the grating increases along with it. The shifting of the center wavelength thus becomes more efficient, and less thermal energy can be applied to produce the same thermo-optic tilt of the output waveform. In this document, such patterned heaters will be referred to as “efficient patterned heaters.”
Optical integrated circuits may contain arrayed waveguide gratings, waveguides, optical couplers, splitters, three dimensional optical memory devices, Bragg gratings, optical attenuators, optical splitters, optical filters, optical switches, lasers, modulators, interconnects, optical isolators, optical add-drop multiplexers (OADM), optical amplifiers, optical polarizers, optical circulators, phase shifters, optical mirrors/reflectors, optical phase-retarders, optical detectors, and other optical components. It should be understood that optical integrated circuits are not ‘circuits’ in the strict sense (e.g., they do not have ‘ground’ paths), although they are commonly called optical integrated circuits by analogy to electrical integrated circuits. As in the case of electrical (semiconductor) integrated circuits, fabrication of several optical components on a shared substrate results in several advantages, including substantial cost savings during manufacture, and smaller size of the assembly. Multi-device optical integrated circuits built with planar waveguides on common substrates are known as “planar lightwave circuits,” or PLCs.
In order to fabricate an AWG directly to the 100 GHz optical frequency grid commonly used for DWDM, the accuracy of absolute index of refraction would need to be better than 10−5. This level of process control is presently difficult to achieve. As has already been discussed, actual manufacturing variations can be compensated for by adjusting the selected operating temperature for active stabilization. It is unlikely that two AWG's on the same chip would require the same stabilization temperature. Given the currently employed methods of wavelength stabilization, it is challenging to control thermally more than one grating on a given PLC.